1. Field of the Invention
This invention relates to an optical scanning apparatus in optical equipment using such an optical system such as a laser beam printer and so on, and more particularly to an optical scanning apparatus which gives an F-theta (F-.theta.) function by means of an axis-symmetric aspherical lens and facilitates the manufacture, assembly and adjustment of a lens by providing a cylindrical lens between a rotating polygon mirror and the axis-symmetric aspherical lens.
2. Description of the Prior Art
Recently, optical equipment using a laser, for example a laser printer, laser scanning micrometer (LCM), pattern generator, etc., has been developed. Ordinarily, an optical system applying a laser is composed of a laser generator, a laser deflector, a laser scanning lens system, and a light receiving portion.
In addition, in a image forming apparatus such as copy machines and printers, etc., use a electronic photograph process method which frequently employs a monochromatic laser beam. Namely, such an apparatus configured to form an electrostatic latent image onto a photosensitive object by leading a laser beam emitted from a laser beam emitting device such as a semiconductor laser diode to a polygon deflecting mirror rotating at high speed as parallel light through a collimator lens on a prism complex, etc., and thereafter changing reflection direction of the laser beam by the rotation of the deflecting mirror.
An example of such an optical scanning apparatus is disclosed in Japanese Patent Laid-Open Publication No. Hei5 (1990)-161410 which will be hereafter explained as prior art with reference to FIG. 1.
Referring to FIG. 1, the laser beam emitted from a source of light 1 is scanned onto an image formation plane (photosensitive drum surface) 9 by way of a rotating asymmetric aspherical lens F-.theta. 6' for scanning the laser beam is composed of a first lens 6 with a rotating symmetric axis whose first plane and second plane are a spherical plane on a flat plane and a second lens 7 whose third plane is the cylindrical lens and whose fourth lens is the rotating asymmetric aspherical lens for correcting aberration.
In addition, the trace of the spot in the image formation plane 9 moves uniformly by the first plane and second plane of the scanning lens 6' when the rotating polygon mirror 5 is rotated with equal angles.
On the other hand, the effect of vibration of the rotating polygon mirror 5 is compensated by the third cylindrical plane and fourth aspherical plane of the second lens 7 located between the rotating polygon mirror 5, the first lens 6 and the image formation plane 9.
However, the prior art of FIG. 1 has problems as follows.
Firstly, the size of the lens must be large because the third plane and the fourth plane are located between the scanning lens and the image formation plane, whereby the lens may be greatly influenced by contraction/expansion due to humidity and temperature and the size of the entire optical system must be large.
Secondly, because the third plane and the fourth plane are consist of the cylindrical lens and the aspherical lens respectively, even if they are manufactured by using a mould, the yield is likely to decrease when manufacturing and assembling the lens as it is difficult to properly align the two plane axes.